1. Field of the Invention
The present invention relates to a drive circuit for a stepping motor and, more particularly, to a drive circuit for a stepping motor, which employs a method, such as a 1-2 phase excitation mode, wherein 1-and 2-phase excitation modes are alternately repeated.
2. Related Background Art
In a stepping motor, pole pieces of a stator having a plurality of excitation coils and magnetic poles of a rotor are arranged to face each other. A pulse current (excitation current) is supplied to the excitation coils in the predetermined order to sequentially rotate the rotor by a predetermined angle (step angle).
Therefore, a rotational angle (amount of motor rotation) of the rotor is proportional to the number of pulses of the drive current, and a rotational speed is proportional to its frequency.
Drive circuit modes of a stepping motor of this type include a 1-phase excitation mode for sequentially exciting a motor by one phase, a 2-phase excitation mode for sequentially exciting a motor by two phases, and a 1-2 phase excitation mode for alternately repeating 1- and 2-phase excitation modes.
In the 1- and 2-phase excitation modes, a motor can always be driven by a constant excitation current even if a phase is switched. Therefore, an average value of a torque is theoretically constant.
In the 1-2 phase excitation mode, since the 1- and 2-phase excitation modes alternately appear, a large variation in torque (theoretically, 41%) occurs, resulting in vibration and noise. This is because a drive current in the 2-phase mode for energizing two coils is larger than (about twice) a drive current in the 1-phase mode for energizing one coil, and an output torque varies in proportion thereto.
Another drive method has been disclosed in Japanese Patent Laid-Open No. 156195. In this method, an output torque is maintained constant by controlling a supply current. Then step-out is prevented even if changes in gravity or static friction occur due to variations in load or aging.
In the conventional method, a supply current is controlled to stabilize an output torque. The method is performed when a pulse rate is changed to change a speed. Therefore, when a motor is driven at a given constant speed between high and low speeds, a constant current flows. Thus, in the 1- and 2-phase excitation modes, a torque is kept constant, as described above. However, in the 1-2 phase excitation mode, even if a constant current is supplied, a variation in torque undesirably occurs.
The conventional methods aim at stabilization of a torque when the speed of the motor is changed but cannot eliminate variations in torque when the 1- and 2-phase modes are switched in the 1-2 phase excitation mode.
U.S. Pat. No. 3,757,193 discloses a drive circuit for a pulse motor, which switches excitation currents. However, in this patent, excitation currents are switched in rotating and stationary states of the rotor, but cannot eliminate variations in torque when the 1- and 2-phase modes are switched in the 1-2 phase excitation mode.